The present invention relates to a semiconductor light-emitting device such as light-emitting diodes to be used for display, optical communications and the like, and also to a manufacturing method therefor. The invention further relates to an LED lamp and an LED display, whichever is equipped with such a semiconductor light-emitting device.
In recent years, there have been developed high-intensity light-emitting diodes (LEDs) which emit light of infrared to blue wavelengths. This is based on the fact that the crystal growth technique for direct-transition group III-V compound semiconductor materials has been improved dramatically so that crystal growth has become implementable for almost any semiconductor that belongs to the group III-V compound semiconductors. LEDs using these direct-transition materials, by virtue of their capability of high-output, high-intensity emission, have come to be widely used as high-intensity LED lamps such as outdoor display boards, display-use light sources such as indicator lamps for portable equipment of low power consumption, and light sources for optical transmission and optical communications by plastic optical fibers.
As a new high-output, high-intensity LED of this type, there has been known an LED using AlGaInP-based material as shown in FIG. 11. This LED is fabricated by the following process. That is,
On an n-type GaAs substrate 1, are stacked one after another:                an n-type GaAs buffer layer 2;        a distributed Bragg reflector layer (dopant concentration: 5×1017 cm−3) 4 made of a multilayer film in which n-type (AlxGa1-x)0.51In0.49P (x=0.45) and n-type Al0.51In0.49P are stacked alternately;        an n-type (AlxGa1-x)0.51In0.49P lower cladding layer (0≦x≦1, e.g. x=1.0; thickness: 1.0 μm; dopant concentration: 5×1017 cm−3) 5;        a p-type (AlxGa1-x)0.51In0.49P active layer (0≦x≦1, e.g. x=0.42; thickness: 0.6 μm; dopant concentration: 1×1017 cm−3) 6; and        a p-type (AlxGa1-x)0.51In0.49P upper cladding layer (0≦x≦1, e.g. x=1.0; thickness: 1.0 μm; dopant concentration: 5×1017 cm−3) 7,        and further thereon are formed:        a p-type (AlxGa1-x)vIn1-vP intermediate layer (x=0.2; v=0.4; thickness: 0.15 μm; dopant concentration: 1×1018 cm−3) 8;        a p-type (AlxGa1-x)vIn1-vP current spreading layer (x=0.05; v=0.05; thickness: 1.5 μm; dopant concentration: 5×1018 cm−3) 10; and        an n-type GaP current blocking layer (thickness: 0.3 μm; dopant concentration: 1×1018 cm−3) 9.        
Thereafter, the n-type GaP current blocking layer 9 is subjected to selective etching by normal photolithography process so that a 50 μm to 150 μm-dia. portion thereof shown in the figure is left while its surrounding portions are removed. A p-type (AlxGa1-x)vIn1-vP current spreading layer (x=0.05; v=0.95; thickness: 7 μm; dopant concentration 5×1018 cm−3) 10 is regrown in such a manner as to cover the top of the p-type (AlxGa1-x)vIn1-vP current spreading layer, which has been exposed by removing the n-type GaP current blocking layer 9, as well as the n-type GaP current blocking layer 9.
Finally, on the p-type current spreading layer 10 is deposited, for example, a Au—Be film. This film is patterned into a circular form, for example, so as to be inverse to the light-emitting region, thereby forming a p-type electrode 12. Meanwhile, on the lower surface of the GaAs substrate 1 is formed, for example, an n-type electrode 11 made of a Au—Zn film by deposition.
It is noted here that, for simplicity' sake, the ratio x of Al to Ga, the ratio v of totaled Al and Ga to the other group III elements, or the like will be omitted as appropriate in the following description.
With respect to the p-type AlGaInP current spreading layer 10, the Al composition “x” and the In composition (1-v) are set low, as already described, so that the current spreading layer becomes transparent to the emission wavelength range 550 nm-670 nm of this AlGaInP-based LED, low in resistivity, and makes ohmic contact with the p-side electrode (i.e., x=0.05, v=0.95). In the AlGaInP-based LED, normally, Si is used as the n-type dopant, and Zn is used as the p-type dopant. Also, the conductive type of the active layer is normally the p type.
As the substrate for (AlxGa1-x)vIn1-vP-based LEDs, normally, a GaAs substrate is used so as to obtain lattice matching with materials of individual layers. However, the GaAs substrate has a band gap of 1.42 eV, lower than those of (AlxGa1-x)vIn1-vP-based semiconductors, so that the GaAs substrate would absorb light emission of 550 nm to 670 nm, which is a wavelength range of (AlxGa1-x)vIn1-vP-based semiconductors. Therefore, out of light emitted from the active layer, the light emitted toward the substrate side would be absorbed within the chip, and could not be extracted outside. Accordingly, for (AlxGa1-x)vIn1-vP-based LEDs, with a view to fabricating a high-efficiency, high-intensity LED, it is important to provide a DBR (distributed Bragg reflector) layer 4 in which low-refractive-index layer and high-refractive-index layer are combined one after another between the GaAs substrate 1 and the active layer 6 as shown in FIG. 11 so as to obtain an enhanced reflectance through multiple reflection. In this example of FIG. 11, (Al0.65Ga0.35)0.51In0.49P (refractive index: 3.51) that does not absorb the emission wavelength 570 nm of the active layer is selected as the high-refractive-index material, and Al0.51In0.49P (refractive index: 3.35) is selected as the low-refractive-index material, while optical film thicknesses of the individual low-refractive-index layer and high-refractive-index layer are set to λ/4 relative to an emission wavelength of λ. These materials are stacked alternately to an extent of 10 pairs so as to be enhanced in reflectance, by which the total photoreflection-layer reflectance is set to about 50%. In a case where such an AlGaInP-based light-reflecting layer is provided, reflectance characteristics against the number of pairs are shown in FIG. 13A. The expression “AlInP/Q(0.4)” in the figure indicates a characteristic with the use of a pair of (Al0.65Ga0.35)0.51In0.49P and Al0.51In0.49P. Similarly, the expression “AlInP/Q(0.5)” indicates a characteristic with the use of a pair of (Al0.55Ga0.45)0.51In0.49P and Al0.51In0.49P With this light-reflecting layer adopted, the chip luminous intensity can be improved from 20 mcd to 35 mcd, compared with the case where no light-reflecting layer is provided.
As is well known, if the layer thickness of crystals is “d” and the refractive index is “n,” then the optical film thickness is given by “nd.”
As shown in FIG. 12, in (AlxGa1-x)vIn1-vP-based LEDs is used a light-reflecting layer 14 which is formed by stacking a pair of AlxGa1-xAs and AlAs and which has lattice matching with the GaAs substrate. In a case where such an AlGaAs-based light-reflecting layer 14 is provided, reflectance characteristics against the number of pairs are shown in FIG. 13B. In the figure, a broken line expressed as “Al0.60” shows a characteristic with the provision of a light-reflecting layer which is formed by selecting Al0.65Ga0.35As (refractive index: 3.66), which does not absorb the emission wavelength 570 nm of the active layer, as the high-refractive-index material and selecting AlAs (refractive index: 3.10) as the low-refractive-index material, and then stacking alternately these materials as a pair. Similarly, a broken line expressed as “Al0.70” in the figure shows a characteristic with the use of a pair of Al0.70Ga0.30As and AlAs, and a solid line expressed as “Al0.75” in the figure shows a characteristic with the use of a pair of Al0.65Ga0.35As and AlAs. As a result of this, in the case where Al0.65Ga0.35As (refractive index: 3.66) is selected as the high-refractive-index material and AlAs (refractive index: 3.10) is selected as the low-refractive-index material, it becomes possible to provide a larger difference in refractive index than in the case shown in FIG. 13A, where the total reflectance of the light-reflecting layer can be made to be about 60%. With this light-reflecting layer adopted, the chip luminous intensity can be improved from 20 mcd to 40 mcd, compared with the case where no light-reflecting layer is provided.
In this connection, as can be understood from FIGS. 13A and 13B, in order to obtain a high reflectance of 90% or more, which allows the luminous intensity to be improved double or more, the number of semiconductor layer pairs constituting the light-reflecting layer 4, 14 needs to be 30 or more. This is due to very small differences in refractive index, which are 0.18 in the case of (AlxGa1-x)0.51In0.49P(x=0.45)/Al0.51In0.49P and 0.32 or so in the case of AlAs/AlxGa1-xAs.
However, providing a pair number of 30 or more would cause the growth time to be prolonged, which would lead to lower mass-productivity. Also, small differences in refractive index would cause the half-value width of the reflection spectrum to be narrowed, where only a slight change in layer thickness of the light-reflecting layer would cause the reflection spectrum to be largely shifted, making it difficult to obtain a matching between emission wavelength and light-reflecting layer, which would lead to lower reproducibility and so lower mass-productivity. Further, with the number of pairs increased, the light-reflecting layer alone would take a layer thickness as thick as 3 μm or more, where the substrate after epitaxial growth would be liable to warp or deformation, making it difficult to subject the substrate to subsequent processes.
These circumstances are the same also with semiconductor light-emitting devices using other various materials without being limited to the AlGaInP-based materials.